'\" et
.TH MSGSND "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
msgsnd
\(em XSI message send operation
.SH SYNOPSIS
.LP
.nf
#include <sys/msg.h>
.P
int msgsnd(int \fImsqid\fP, const void *\fImsgp\fP, size_t \fImsgsz\fP, int \fImsgflg\fP);
.fi
.SH DESCRIPTION
The
\fImsgsnd\fR()
function operates on XSI message queues (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fImsgsnd\fR()
function shall send a message to the queue associated with the
message queue identifier specified by
.IR msqid .
.P
The application shall ensure that the argument
.IR msgp
points to a user-defined buffer that contains first a field of type
.BR long
specifying the type of the message, and then a data portion that holds
the data bytes of the message. The structure below is an example of
what this user-defined buffer might look like:
.sp
.RS 4
.nf

struct mymsg {
    long   mtype;       /* Message type. */
    char   mtext[1];    /* Message text. */
}
.fi
.P
.RE
.P
The structure member
.IR mtype
is a non-zero positive type
.BR long
that can be used by the receiving process for message selection.
.P
The structure member
.IR mtext
is any text of length
.IR msgsz
bytes. The argument
.IR msgsz
can range from 0 to a system-imposed maximum.
.P
The argument
.IR msgflg
specifies the action to be taken if one or more of the following is
true:
.IP " *" 4
The number of bytes already on the queue is equal to
.BR msg_qbytes ;
see
.IR <sys/msg.h> .
.IP " *" 4
The total number of messages on all queues system-wide is equal to the
system-imposed limit.
.P
These actions are as follows:
.IP " *" 4
If (\fImsgflg\fP & IPC_NOWAIT)
is non-zero, the message shall not be sent and the calling thread
shall return immediately.
.IP " *" 4
If (\fImsgflg\fP & IPC_NOWAIT) is 0, the calling thread shall suspend
execution until one of the following occurs:
.RS 4 
.IP -- 4
The condition responsible for the suspension no longer exists, in which
case the message is sent.
.IP -- 4
The message queue identifier
.IR msqid
is removed from the system; when this occurs,
.IR errno
shall be set to
.BR [EIDRM] 
and \-1 shall be returned.
.IP -- 4
The calling thread receives a signal that is to be caught; in this case
the message is not sent and the calling thread resumes execution in the
manner prescribed in
.IR "\fIsigaction\fR\^(\|)".
.RE
.br
.P
Upon successful completion, the following actions are taken with
respect to the data structure associated with
.IR msqid ;
see
.IR <sys/msg.h> :
.IP " *" 4
.BR msg_qnum
shall be incremented by 1.
.IP " *" 4
.BR msg_lspid
shall be set to the process ID of the calling process.
.IP " *" 4
.BR msg_stime
shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.SH "RETURN VALUE"
Upon successful completion,
\fImsgsnd\fR()
shall return 0; otherwise, no message shall be sent,
\fImsgsnd\fR()
shall return \-1, and
.IR errno
shall be set to indicate the error.
.SH ERRORS
The
\fImsgsnd\fR()
function shall fail if:
.TP
.BR EACCES
Operation permission is denied to the calling process; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EAGAIN
The message cannot be sent for one of the reasons cited above and
(\fImsgflg\fP & IPC_NOWAIT) is non-zero.
.TP
.BR EIDRM
The message queue identifier
.IR msqid
is removed from the system.
.TP
.BR EINTR
The
\fImsgsnd\fR()
function was interrupted by a signal.
.TP
.BR EINVAL
The value of
.IR msqid
is not a valid message queue identifier, or the value of
.IR mtype
is less than 1; or the value of
.IR msgsz
is greater than the system-imposed limit.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Sending a Message"
.P
The following example sends a message to the queue identified by the
.IR msqid
argument (assuming that value has previously been set). This call
specifies that an error should be reported if no message is available.
The message size is calculated directly using the
.IR sizeof
operator.
.sp
.RS 4
.nf

#include <sys/msg.h>
\&...
int result;
int msqid;
struct message {
    long type;
    char text[20];
} msg;
.P
msg.type = 1;
strcpy(msg.text, "This is message 1");
\&...
result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), IPC_NOWAIT);
.fi
.P
.RE
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess communication
(IPC). Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fImq_close\fR\^(\|)",
.IR "\fImq_getattr\fR\^(\|)",
.IR "\fImq_notify\fR\^(\|)",
.IR "\fImq_open\fR\^(\|)",
.IR "\fImq_receive\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImq_setattr\fR\^(\|)",
.IR "\fImq_unlink\fR\^(\|)",
.IR "\fImsgctl\fR\^(\|)",
.IR "\fImsgget\fR\^(\|)",
.IR "\fImsgrcv\fR\^(\|)",
.IR "\fIsigaction\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue",
.IR "\fB<sys_msg.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
